1. Field of the Invention
This invention relates to determining the concentration of oxygen in a gaseous atmosphere.
2. Prior Art
U.S. Pat. Nos. 3,907,657 Heijne and 3,514,377 to Spacil et al relate to the measurement of oxygen (O.sub.2) concentrations using solid electrochemical devices. For applications at elevated temperatures (&gt;500.degree. C.), for example, as might be encountered in the exhaust gases of furnaces or automobiles, the active material in these devices may be ceramic zirconium dioxide suitably adapted for the conduction O.dbd. ions. Electrochemical cells made from this material are suitable at elevated temperatures for oxygen sensing and pumping applications.
The mode of operation of the Heijne device can be described as an oxygen counting mode in which oxygen partial pressure is determined on a sampling basis. A constant current (or equivalent means) is applied to an electrochemical cell which forms part of the enclosure of a volume for a period of time, t.sub.p, for the purpose of electrochemically pumping out most of the oxygen from that volume. The ambient atmosphere had established itself within the volume prior to the pump out, by means of a leak. An additional electrochemical cell, which serves as a sensor of the reduced oxygen partial pressure within the volume and which also constitutes a portion of the enclosure, provides a signal indicating when oxygen has been sufficiently depleted from the volume (see FIG. 4 of Heijne). Knowing the temperature, enclosed volume and the pump out current and time allows one to calculate the number of oxygen molecules within the enclosure from the ideal gas law. The number of oxygen molecules is in turn proportional to the desired oxygen partial pressure. If a constant pump current is used, the pump out time t.sub.p is proportional to the oxygen partial pressure. If a constant current is not used, then the integral of the pump out current over the pump out time is proportional to the oxygen partial pressure.
The Heijne device can provide an output which is linearly proportional to the oxygen partial pressure. This is superior, for example, to single oxygen concentration cells used as sensors which give an output (EMF) proportional to the natural logarithm of the oxygen partial pressure ln (P.sub.O.sbsb.2).
A potential disadvantage of the Heijne device is response time. For this measurement procedure, the leak connecting the ambient to the enclosed volume must be small so that during the pump out of oxygen, no significant amount of oxygen leaks into the volume to cause an error in the count of molecules (i.e., to erroneously increase t.sub.p). However, if the leak is made small, it may take a long time, t.sub.v, for the ambient to reestablish itself within the volume after a pump out. If the changes in the oxygen partial pressure in the ambient occur rapidly with respect to this refill time, the device would not be able to follow these changes in repetitive operation.
U.S. Pat. Nos. 3,923,624 to Beckmans et al, 3,654,112 to Beckmans et al, and 3,907,657 to Heijne et al describe tubular ceramic structures for measuring and controlling the composition of oxygen in a carrier gas. In some cases a pump cell and a sensor cell are used. U.S. Pat. Nos. 3,923,624 and 3,654,112 teach devices to be used primarily to dose a gas with oxygen to a constant partial pressure. Measurement of the dosed gas is made by a standard technique using a zirconium dioxide oxygen concentration cell to be sure that the dosed gas contains the correct amount of oxygen. The sensitivity of the concentration cell to the oxygen partial pressure is low, being proportional to ln (P.sub.0.sbsb.2). However, various types of automobile engines, e.g., diesel, do not require that much sensitivity when determining oxygen concentration for use in engine control. Since less sensitivity is acceptable, the additional sensitivity represents an undesirable additional expense. It would be desirable to reduce the cost of the oxygen sensor by not providing more accuracy than is required.
In the case of the teachings of U.S. Pat. No. 3,698,384 to Jones, the purpose is to measure oxygen partial pressure in a feedgas. This is done by measuring an electrochemical cell pumping current while holding the sensor cell voltage a constant. However, to achieve a result in the disclosed open ended tubular structure made from zirconium dioxide the flow rate of the feedgas must be kept constant. If the flow rate should attempt to vary, there is a relatively elaborate flow control circuit to keep the flow rate a constant. This scheme, which also employs a reference atmosphere is relatively unsuitable for application in an auto exhaust where the exhaust flow rate would change substantially with RPM.
U.S. Pat. Nos. 3,347,735 to McKee and 3,857,771 to Sternberg both describe oxygen sensing procedures or devices wherein the taking of a first derivative of an output signal either determines the oxygen partial pressure or can yield information on the medium which contains the oxygen. Neither device would be suitable for the continuous or repeated determination of the oxygen partial pressure in a variable, high temperature environment like that occurring in an automotive exhaust. U.S. Pat. Nos. 3,948,081; 3,738,341 and 4,112,893 relate to oxygen sensors and associated electrical circuitry which are a typical oxygen concentration cell type. These patents discuss external circuitry which may enhance the operation of such sensors under various conditions.
An important application of high temperature oxygen sensors is in the determination of the stoichiometric air fuel mixture in the exhaust gases of hydrocarbon fired furnaces or engines such as automobile internal combustion engines. The stoichiometric mixture is one in which the mass of air present contains just enough oxygen to react with the mass of hydrocarbons present so that there is the minimum amount of both oxygen and hydrocarbons remaining. For common automotive gasoline, the air fuel ratio A/F(=mass of air/mass of fuel) at the stoichiometric point is approximately 14.6. If, for example, an engine were running lean of stoichiometry (A/F&gt;14.6) there would be an excess of air in the "charge" burned in the cylinder and the exhaust gas would contain a substantial oxygen partial pressure. If rich operation were occurring (A/F&gt;14.6), the exhaust gas would contain unreacted or partially reacted hydrocarbons and very low oxygen partial pressure. In particular, the equilibrium oxygen partial pressure in the exhaust gas can change by a great amount (as much as 20 orders of magnitude) as one moves from lean to rich operation. This large change forms the basis for detecting the stoichiometric air fuel ratio with an exhaust gas oxygen sensor. The electrical output of such a sensor can then be fed back to an electrically controllable carburetor or fuel injection system for maintaining engine operation always at the stoichiometric point. Depending on engine type operation at this point frequently offers a reasonable compromise for minimizing regulated exhaust gas emissions and maximizing engine performance.
There are known high temperature oxygen sensors utilizing oxygen electrochemical concentration cells (usually made from zirconium oxide) and requiring the use of a reference atmosphere (usually air) are are suitable for determining the stoichiometric air fuel ratio in a high temperature automotive environment. These devices given an output (EMF) proportional to the natural logarithm of the oxygen partial pressure. Despite their low sensitivity to oxygen partial pressure, the large change in oxygen partial pressure at the stoichiometric point allows their useful implementation.
For some engines it is useful to operate lean of the stoichiometric A/F for the purpose of reducing fuel consumption. Oxygen partial pressure varies in a systematic way in the lean region and this can form the basis for determining lean A/F. Knowledge of lean A/F would be useful to fully implement a lean burn engine strategy which would maximize fuel economy and engine performance and minimize regulated emissions. However, the variation in oxygen partial pressure in the appropriate lean A/F region, e.g., 16&lt;A/F&lt;20, is not large, (in comparison to the changes occurring near stoichiometry) so that suitable oxygen sensors with sensitivities greater than the natural logarithm of oxygen partial pressure are desirable for accurate measurement in the desired A/F range. For some automobile engines a step function output may provide sufficient sensitivity, as long as the step occurred within the desired lean A/F range. Known step function outputs occur at stoichiometry, but not at other A/F values. These are some of the problems this invention overcomes.
Also known is a patent application entitled "Steady State Mode Oxygen Sensor", Ser. No. 126,606, filed on Mar. 3, 1980 by the inventors of this invention, which discloses quantitative measurement of oxygen partial pressure in exhaust gases. In accordance with that patent application, a ceramic electrochemical structure with associated external circuitry is capable of measuring oxygen concentration in a high temperature surrounding environment such as may be found in an automotive exhaust. The external circuitry provides an electrical output whose magnitude is proportional to the percentage of gaseous oxygen. The structure includes two oxygen ion (O.dbd.) conducting electrochemical cells, a pump cell and a sensor cell, which in part provide the enclosing structure of a nearly enclosed volume. A portion of the remaining structure can be a hollow ceramic tube. The cells are attached to the end faces of the tube. A small aperture in the enclosing structure allows the ambient atmosphere, containing oxygen in a percentage to be determined, to leak into the volume.
In operation, the external circuitry causes a voltage to be applied to the pump cell with a propoer polarity to electrochemically pump oxygen out of the volume and return it to the ambient. After a brief transient period, a steady state is reached where the rate at which oxygen is pumped from the volume is equaled at the rate at which oxygen is diffusing into the volume by means of the aperture. Under this steady-state condition, the oxygen partial pressure within the volume is reduced from that in the ambient causing an EMF to develop across the electrodes of the sensor cell. Experimentally, it is found that if one causes the pump cell current to be continuously adjusted so that the sensor cell EMF if always a constant, then the magnitude of the pump cell current is linearly proportional to the percentage of oxygen in the ambient atmosphere. This linear relationship is the basis of sensor operation.
The above described oxygen sensor requires two Nernst cells, the pump cell and the sensor cell. It would be advantageous to develop a circuit which requires only one Nernst cell thus obviating the need for the seond cell and the associated seals coupling the cell to an enclosed volume. Further, the above devices disclosed operate on quantitative electrical measurements. As a result, components in such a system are relatively more critical than a system which would operate on qualitative electrical characteristics. For example, a step function output positioned at a desired lean A/F operating point can be classified as a qualitative signal and yet provide a sufficiently accurate output for certain engine types. These are some of the problems this invention overcomes.